THE METHYL-ACCEPTING TRANSDUCER PROTEIN HTRI IS FUNCTIONALLY ASSOCIATED WITH THE PHOTORECEPTOR SENSORY RHODOPSIN-I IN THE ARCHAEON HALOBACTERIUM-SALINARIUM

We have investigated the functional relationship between two proteins involved in the photosensory system of the archaeon Halobacterium sali narium: the photoreceptor sensory rhodopsin I (SRI) and the halobacter ial transducer rhodopsin I (HtrI), which has been proposed to be the p utative signal transducer of SRI, by genomic DNA analysis of two indep endent SRI negative mutants, Pho81 and D1. Southern and PCR analyses r evealed that both strains bear alterations in the 5' flanking region o f the gene encoding SRI, sopI. DNA sequence analysis confirmed the occ urrence in this region of htrI, the gene encoding the putative transdu cer protein. PCR and Northern analyses have shown further that sopI an d htrI are expressed as a single transcriptional unit, thus explaining the lack of SRI in mutants with a defective htrI. Expression of the c loned sopI under the control of a heterologous promoter did not restor e the SRI-dependent photoresponse in the strain Pho81. Moreover, the p hotocycling rate of the expressed pigment was clearly lower than in wi ld type. HtrI is therefore essential for SRI function and most likely modulates the photochemical properties of the photoreceptor via direct physical interaction. Finally, reintroduction of both sopI and htrI i nto Pho81 and D1 restored the SRI photochemistry and its physiological function. Our results provide the first experimental evidence for the functional coupling between SRI and HtrI and corroborate the proposed model in which HtrI acts as the signal transducer of this archaeal se ven-helix photoreceptor in a way analogous to the bacterial chemotaxis transducers.